Premixed fuel burner assemblies are used with various heating equipment such as boilers, commercial hot water headers, fuel barbeques, and the like. Fuel burners are devices into which a flow of combustible fuel (usually gas) is introduced into a mixing chamber, where it is mixed with air supplied in a suitable proportion to the combustible gas. After mixing, the mixture of combustible fuel and air exits the mixing chamber through burner ports where it is ignited and burnt.
Specifically, a typical premixed fuel burner assembly consists of a hollow burner body having a closed end and an open end into which the premixed fuel/air flows. The burner body includes a porting area that consists of burner flame port perforations (i.e. slots and/or holes). Within the burner body is a venturi tube that typically contains a multiplicity of holes through and out which the fuel and air mixture from the interior of the body flows. Fuel and air are both provided into the boiler body through the venturi tube. Specifically, fuel is provided into the venturi tube through a fuel nozzle and air is provided around the fuel nozzle. Fuel and gas are mixed to produce a combustible mixture which subsequently is passed through the burner body and ignited to produce a burner flame that, in the case of a water heater is applied to a heat exchanger of the boiler.
Conventional premixed fuel burner assemblies produce short flames that are just beyond or above the burner porting area. Normally the mixture has 30 percent excess air so as to provide cleaner combustion products. At loadings (i.e. heat per unit area) below approximately 6 kilowatts per square decimeter, the burner port surface will be radiant since the velocity of the mixture is low resulting in the flame being positioned on or closely adjacent to the surface. This gives rise to problems of thermal fatigue and high temperature oxidation of the burner porting surface, and potential flashback of the flame into the burner body. At higher loadings (e.g. 12 kilowatts per square decimeter and above) the increase in volumetric flow is such that the velocity of the mixture may be increased to the point where the flame front is further from the burner porting surface resulting in a relatively cool porting surface. However, at high loadings if the amount of excess air is not or cannot be controlled, overheating of burner porting surface can still result when there is inadequate excess air (i.e. when there is too much fuel in the air/fuel mixture) since in such a case the flame will sit on the surface of the burner increasing the temperature. As is conventionally known, when the burner body becomes too hot (e.g. 2000 degrees Celcius) the fuel burner assembly can suffer failures, melting and irreversible damage.
Various types of fuel burner assemblies have been developed to attempt to maintain the flames above the surface of the outer cylinder by operating at high loadings (i.e. high fuel/air velocities) while at the same time maintaining a proper mix of fuel and flame stability. For example, U.S. Pat. No. 6,461,152 to Wood et al. discloses a tubular burner consisting of a cylindrical tubular body into which a distributor component can be fitted. The distributor is substantially the same axial length as the tubular burner body but of a smaller cross-sectional dimension than said body so to allow for easy insertion. The distributor divides the burner body into an upper and a lower chamber. The distributor has a first tubular portion and a second extension portion each of which is provided with axially aligned flanges having a number of perforations. While this assembly achieves a reasonable distribution of air and fuel streams prior to delivering the fuel/air mixture to the porting area of the tubular burner, the construction and assembly of this burner is expensive, as is the manufacture of the various components involved in the construction.